Studying the temporal and spatial variation of water quality in a reservoir is beneficial for forecasting water eutrophication and preventing algal bloom. Taking Yuqiao Reservoir (YQR) as a case in this paper, the relationship between the inlet water quality -temperature, total nitrogen (TN), total phosphorus (TP) and the inlet discharge was analysed according to the observed data from 2011 to 2015. The response of the water quality in YQR to the inlet discharge was also modelled during the 2012, by utilizing a 2D hydrodynamic and water quality model. The results indicate that from November to April, the water temperature is controlled by the atmosphere, and there is no obvious difference between the inlet temperature and the temperature in YQR. From May to October, during non-water-diversion period, the temperature in the upstream of YQR is the highest. During water-diversion period, the temperature difference presents a linear increasing trend along the inlet discharge, and the temperature in the upstream of YQR decreases and reaches the minimum value. The variation of the nutrients (TN & TP) concentrations is basically consistent throughout the year. During non-water-diversion period, there is a high concentration of TN with a low concentration of TP in Guoheqiao. Meanwhile, the nutrients gather in Guohe section, but decay in YQR with a small concentration gradient. During water-diversion period, the TN concentration presents a decreasing trend of power function, but TP concentration presents a linear rising trend. At the same time, the nutrients are transported to the upstream of YQR, which causes the TN & TP concentrations and the concentration gradient increasing in YQR. In addition, the minimum value of water temperature and the peak values of TN & TP concentrations lag behind the change of the discharge from Guohe River. The south bank is more vulnerable to the incoming flow of Guohe River than the north bank, where has suffered more serious pollution.